1. Technical Field of the Invention
This invention relates generally to injection molding and more particularly to a nozzle for injection molding machines
2. Discussion of the Related Art
A nozzle for an injection molding machine is described in West German Patent DE-OS 21 06 546. However, this nozzle has as an output channel and only one blowing needle which is Permanently connected to the nozzle and has a constant length. Accordingly, when the mold is closed, this nozzle always extends to the fluid center of the plastic previously injected by means of another nozzle and thus leaves an opening or at least--if the second medium is another plastic component--a distinct disturbance of the surface in the extruded plastic article which is apparent when the mold is opened after the first injected plastic has set. This problem is also present in the identical embodiments of the injection molding machine described in FIGS. 1 and 2 of GB-PS 2 139 548.
Such openings or disturbances of the surface are undesirable in many injection molded plastic articles. To correct the imperfections, the articles must be subjected to another after-treatment. Moreover, this problem is also present in injection molded plastic articles which are produced with a generic nozzle according to the British patent application GB 2 202 181. This generic nozzle has a single axially movable blowing needle wherein the interior of the blowing needle forms the supply line which is provided with a non-return valve and supplies a second medium to the mold cavity. The outside of the blowing needle and the inside of the cavity of the nozzle body in the withdrawn state form an annular gap in the region of the nozzle orifice which--when the second medium is a compressed gas or a liquid under pressure--serves solely to relieve the pressure of the second medium after the first injected plastic has set.
The problem of the residual opening or at least distinct disturbance of the surface is also not eliminated by the use of two component nozzles that are no longer generic as disclosed in US-PS 4 101 617, DE-PS 28 00 482 or the embodiment of FIG. 5 in the GB-PS 2 139 548. During the injection process, either an axially movable or permanent blowing needle is used whereby a cylindrical central strand comprising the second medium is for a time at least enclosed by a hollow cylindrical strand comprising the polymer melt forming the outer skin of a plastic article. In the case where the pressure of the second medium is maintained by the nozzle in the mold and runner until the outer polymer melt has cured, an opening or a distinct disturbance of the surface of the plastic article remains in the runner itself after the removal of the runner (cf. for example FIGS. 1-3, 1-4, and 2-1 to 2-3 in the DE-PS 28 00 482 and the embodiment in FIG. 5 of the GB-PS 2 139 548). In the case where after the construction of the plastic article interior which is filled with the second medium and a stopper of the outer polymer melt is pushed into the mold cavity (cf. FIG. 3 of the US-PS 4 101 617) and the second medium is a compressed gas or a liquid under pressure, another opening must be punched or drilled into the plastic article from the outside before or even after opening the mold for safety reasons (cf. also FIGS. 2 and 3 of US-PS 4 101 617). In addition to these problems, two component nozzles have a significantly more complicated design than nozzles for a single component of an injection molding unit which processes several components simultaneously. Accordingly, in many cases it is more cost-effective and requires less maintenance to equip an injection molding unit for the simultaneous processing of several components with several nozzles for each component to be injected than with a single multi-component nozzle.
Therefore, it is an object of the present invention to Provide a nozzle for injection molding machines which introduces a second medium into the interior of a first polymer melt that is already partially or completely injected into an injection mold, wherein with minimum design this nozzle permits the seamless closing of the lead-in opening for the second medium through the first polymer melt.
It is a further object of the present invention to accomplish the foregoing object while permitting the relief of Pressure in the interior of the plastic article within the closed mold without an external opening of the plastic article when the second medium is a compressed gas or liquid under pressure.
Other objects and advantages will become apparent from the specification and drawings which follow.